Add contributing.md file

[KrzysztofMadejski]

• explaining pull request process
• guidelines for issues
• translation process

Some examples:

• https://gist.github.com/PurpleBooth/b24679402957c63ec426
• https://github.com/puppetlabs/puppet/blob/master/CONTRIBUTING.md
• https://github.com/thoughtbot/factory_girl_rails/blob/master/CONTRIBUTING.md
• https://github.com/FortAwesome/Font-Awesome/blob/master/CONTRIBUTING.md

[KrzysztofMadejski]

Closed in #338